The neurobiological mechanisms and therapeutic prospect of extracellular ATP in depression

Abstract Background Depression is a prevalent psychiatric disorder with high long‐term morbidities, recurrences, and mortalities. Despite extensive research efforts spanning decades, the cellular and molecular mechanisms of depression remain largely unknown. What's more, about one third of patients do not have effective anti‐depressant therapies, so there is an urgent need to uncover more mechanisms to guide the development of novel therapeutic strategies. Adenosine triphosphate (ATP) plays an important role in maintaining ion gradients essential for neuronal activities, as well as in the transport and release of neurotransmitters. Additionally, ATP could also participate in signaling pathways following the activation of postsynaptic receptors. By searching the website PubMed for articles about “ATP and depression” especially focusing on the role of extracellular ATP (eATP) in depression in the last 5 years, we found that numerous studies have implied that the insufficient ATP release from astrocytes could lead to depression and exogenous supply of eATP or endogenously stimulating the release of ATP from astrocytes could alleviate depression, highlighting the potential therapeutic role of eATP in alleviating depression. Aim Currently, there are few reviews discussing the relationship between eATP and depression. Therefore, the aim of our review is to conclude the role of eATP in depression, especially focusing on the evidence and mechanisms of eATP in alleviating depression. Conclusion We will provide insights into the prospects of leveraging eATP as a novel avenue for the treatment of depression.


| INTRODUC TI ON
Depression is a complex, prevalent, and severe mental disease 1 that exerts a detrimental impact on daily activities and diminishes the quality of life for millions of people worldwide.By 2030, it is projected to rank among the top three contributors to the disease burden, 2 and a significant cause of disability and suicidal behavior. 3pression, characterized by low mood, feelings of hopelessness, loss of happiness, and lack of motivation, affects up to 350 million people worldwide. 4,5The probability of women suffering from depression is more than 30% while that of men is more than 15%, thereby imposing substantial social and economic burdens. 6Sadly, about one third of individuals with depression do not have effective anti-depressant treatment. 7,8e pathophysiologic cause of depression is unknown, and currently, there are no clinically useful biological diagnostic markers or biological screening tests available. 9However, several recognized risk factors should be taken into consideration, such as low socioeconomic status; comorbid chronic medical conditions, such as diabetes, cardiovascular disease, or obesity; and a personal or family history of major depressive disorder (MDD). 9MDD, is a highly prevalent subtype of clinical depression, and accounts for 4.4% of the global disease burden. 10itial treatment for MDD may involve anti-depressant medication, psychotherapy, or a combination of both.Complementary, alternative, and exercise treatments may also be used but have a more limited evidence base. 11,12Typically, treatment begins with a second-generation anti-depressant (selective serotonin reuptake inhibitor [SSRI]; serotonin-norepinephrine reuptake inhibitor; or atypical anti-depressant like mirtazapine or bupropion). 9However, it is worth noting that the crucial Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study revealed that one third of MDD patients had not achieved remission after four consecutive antidepressant trials. 13Pharmacological treatment resistance to antidepressant therapy is one of the most challenging situations in the clinical management of affective disorders.According to the recent widely accepted definition, treatment-resistant depression (TRD) is characterized by an inadequate response to at least two trials of anti-depressant treatment at adequate dose and duration when administered as monotherapy. 14,15erapeutic strategies to improve inadequate anti-depressant response in TRD patients usually begin with the exclusion of pseudoresistance; Augmentation with second-generation antipsychotics and lithium seem to serve as the treatment options supported by the strongest augmentative evidence in TRD.Nevertheless, the augmentative potential of using anticonvulsive drugs, nutraceuticals, and glutamatergic as well as anti-inflammatory agents remains incompletely verified and needs further research.About other somatic therapies, stimulation therapies, particularly electroconvulsive therapy and repetitive transcranial magnetic stimulation, have been found to be effective in TRD. 16,17So far, intranasal S-ketamine treatment, the sole treatment option specifically indicated in TRD, 18 is an adjunctive therapy to anti-depressants targeting the monoaminergic system.Additionally, many novel investigational drugs and regimens are undergoing clinical trials (see in the review 19 ).
5][26] Positron emission tomography examination has shown the reduced blood flow and impaired glucose metabolism in caudate nucleus, anterior cingulate cortex, and prefrontal cortex of depressed patients. 24,26[33][34] ATP is the biological energy currency and the primary driver of enzyme activity in all cells and tissues. 35Microglia could release large amounts of ATP, especially during neuroinflammation. 20It is broadly recognized that ATP not only supports energy storage within cells but also serves as a rapidly excitatory neurotransmitter or neuromodulator that mediates purinergic signaling in cell proliferation, differentiation, and death, both in healthy individuals and patients with brain disease. 36,37ATP can be released from astrocytes, microglia, neurons, endothelial cells, and smooth muscle cells (present in vessels).Intracellularly, ATP concentration ranges around 5 mM, while in the interstitial space of normal nonstressed tissues, its concentration is in the nanomolar rang. 38,39Under normal circumstances, extracellular ATP (eATP) concentrations are much smaller than intracellular levels, and a state of equilibrium is maintained.In some diseases, eATP is significantly elevated, with concentrations exceeding 100 μ mol/L, far above normal eATP levels.ATP released outside the cell is short-lived and is usually rapidly degraded by ecto-nucleosidase to adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine.
eATP is an important medium of astrocyte-neuron communication and has been linked to depressive-like disorders in rodent models. 40,413][44] Clinical studies on depression also indicate a potential reduction in cellular ATP levels, which could also be observed in rodents with corticotropin-releasing hormones and stress-induced depressive-like behavior. 45Furthermore, studies have shown supplementing ATP can alleviate stressinduced depressive-like behaviors in mice, especially in those with Apolipoprotein E4 (APOE4) targeted replacement. 46It is worth noting that anti-depressants have been reported to increase eATP levels. 47In this review, we focus on summarizing the antidepressant effects of eATP.and also list controversial views aiming to illustrate the critical role of eATP in the occurrence and development of depression.
We have made a  Astrocytes are the most abundant type of glial cells in the brain 32 and play a pivotal role in the pathophysiology of depression. 66eATP serves as a crucial mediator of astrocyte-neuron communication, 72,73 which has been proven to be associated with depressive-like disorders in rodents. 40,415][76] Astrocytes have been reported 8][79][80][81] (2) Astrocytes release ATP mainly through the max-anion channel under hypoxic-glucose deprivation. 824][85][86] In the following sections, we will provide a detailed explanation of the ATP release mechanisms employed by astrocytes and establish the role of eATP in the onset of depression from this perspective.
astrocytes, without dependence on lysosomes and gap junctions. 52udies have shown that intraperitoneal injection of ATP significantly elevates ATP levels in the hippocampus of Calhm2-KO mice and ameliorates the depressive-like behavior observed in these mice.The duration of struggling in the forced swimming test and tail suspension test is prolonged, possibly due to the restoration of the reduced number of dendritic spines. 52Pannexin-1(Panx-1), a plasma membrane channel that allows molecules like ATP to pass through, can have its depressive-like effects blocked through eATP preconditioning when obstructed by gap junction blockers carbenoxolone and mefloquine in the mPFC region. 50This suggests a potential antidepressant role for eATP.However, contrary results have been reported, dye absorption experiments in hippocampal slices showed that acute stress (known to trigger depressive-like behavior) could induce the opening of Connexin 43and Panx-1 channels, leading to ATP release. 87This appears to contradict the previously cited evidence suggesting the anti-depressant effects of ATP.An explanation for these contradictory results may be rooted in the fact that these conclusions are based on a review of different tissue structures and different regions of brain.In vitro research cannot completely simulate the physiological environment in vivo.What's more, different depressive animal models have different pathological bases, which may account for these contradictory findings.

| Itpr2-/IP3R2-Ca 2+ -ATP pathway
Astrocytes are thought to regulate neuronal excitability by releasing ATP in a calcium-dependent manner. 88The inositol 1,4,5-trisphosphate receptors (IP3Rs) located on the endoplasmic reticulum of cells are key mediators in regulating the release of calcium ions. 43The protein encoded by the ITPR2 gene belongs to the IP3R family.Research has demonstrated that both ITPR2 -deletion and blocking vesicular neurotransmitter transmission could lead to inadequate ATP release from astrocytes, resulting in depressive-like behaviors which can be alleviated by supplying ATP and stimulating astrocytes to release endogenous ATP. 41 Additionally, this study reported that it is ATP rather than its hydrolytic product adenosine could reverse depressive-like behaviors in ITPR2 − mice. 41Similarly, another paper suggested that the loss of IP3R2 blocked increased ATP levels in the mPFC of mice exposed to caloric restriction (CR), while this effect was absent in IP3R2-KO mice, 43 implying that the astrocyte IP3R2-ATP pathway may play an important role in CRinduced anti-depressant effects.

| Vesicular nucleotide transporter
Nucleotide transport within cells cannot cross the cell membrane freely. 89Instead, it relies on vesicular storage and transportation. 90me studies suggest that the treatment with SSRI can promote ATP release, resulting in the activation of the ATP receptors via VNUT and the increased brain-derived neurotrophic factor (BDNF) expression in astrocytes. 56Both BDNF 40 and SSRI 56 have been confirmed to be closely related to depression.Furthermore, it is widely acknowledged that Lysosomes in astrocytes contain abundant ATP, 85 and previous studies have shown the crucial role of astrocytic ATP release in depression. 41Soluble epoxide hydrolase, a key enzyme in epoxyeicosatrienoic acid (EET)signal transduction in the mPFC of susceptible mice, is mainly expressed on lysosomes of astrocytes and exhibits an anti-depressant role. 42It is speculated that this may be due to the increased ATP release from astrocytes.
Briefly, there is ample evidence supporting the notion that ATP release from astrocytes could alleviate depression, with only a few studies putting forward the opposite view.The differences may depend on the specific situation.Whatever, these pieces of evidence collectively suggest that ATP release from astrocytes plays an important role in the onset and development of depression.Stimulating the release or replenishment of endogenous ATP in astrocytes may have an anti-depressant effect.Furthermore, studies focusing on how stress reduces ATP release in astrocytes will enhance the comprehension of the pathophysiology of MDD.

| Synaptically-derived ATP and depression
ATP is released through various pathways, which include exocytosis from presynaptic terminals and diffusion through large transmembrane pores (e.g., hemichannels, P2X [7] receptors, or volumesensitive chloride channels) expressed in astroglia membranes. 91In presynaptic terminals, ATP is accumulated and stored in the synaptic vesicles. 91P derived from both glia and neuronal sources can premodulate the efficacy of excitatory synapses and thereby can have an important role in the glia-neuron communications and brain meta-plasticity. 92Synaptic plasticity disturbances are crucial in the onset and progression of depression, especially in the hippocampus, which is highly susceptible to stress and the most frequently studied in the context of depression. 93It is widely acknowledged that depression is considered as a Glial-Based Synaptic Dysfunction, 40 and synaptically-derived ATP, which has the potential to restore synaptic plasticity, may play an important role in depression.

| ATP released from neurons and depression
ATP serves not only as the "energy currency" of neurons, but also as a signaling molecule that facilitates information transfer between neurons and glial cells.ATP functions as a glial transmitter that mediates the neuron-glial cell network interaction. 94eATP can be released from neurons both vesicularly 95,96 and non-vesicularly. 97ATP could exhibit modulatory effects on neuronal functions, with mounting evidence suggesting its pivotal role in synaptic plasticity. 98 Adenosine neuromodulation depends on a balanced activation of inhibitory A1(A1R) and facilitatory A2a receptors (A2aR).Both A1R and A2aR could modulate hippocampal glutamate release and NMDA-dependent long-term potentiation (LTP). 105While A2a receptors are predominantly found in the striatum, they are also present in the limbic system and neocortex. 106,107Their presence in synapses suggests a role for A2a receptors in controlling synaptic transmission, either by presynaptic mechanisms or by regulating NMDA receptor function. 107 is speculated that neuron-derived eATP could potentially play an anti-depressant role by modulating synaptic plasticity.

| ATP+P2XR
9][110][111] These pathways are mediated by the action of nucleoside and nucleotide on P1 (P1R) and P2(P2R) receptors.P1R is a group of 4 different receptors (A1, A2A, A2B. and A3 receptors) and these receptors are all members of the G-proteincoupled family of 7-transmembrane spanning receptors, 112,113 while P2R can be categorized into metabolic (P2Y receptor, P2YR) and ionotropic(P2X receptor, P2XR) subtypes, all of which are sensitive to ATP, ADP, uracil triphosphate and uracil diphosphate. 99,114,115ong purinergic receptors, P2X7R is considered a putative target for therapeutic intervention in mood disorders. 116A1 and A2a receptors are also considered potential targets for therapeutic intervention in mood disorders. 63,64,117,1184.1 | ATP+P2X7R and depression P2X7R, an ATP-gated cation channel, 119 is only activated by a high concentration of eATP (concentration for 50% of maximal effect (EC50) ≥ 100 μmol L −1 ) after exposure to stress.120 When activated, P2X7R allows Ca 2+ and Na + to flow inward and K + to flow out of the cell.121 Stress may trigger excessive ATP release, which in turn activates P2X7R signaling in the hippocampus, potentially promoting the development of depressive-like behaviors.123 This was further supported by another study showing the reduced depressive-like behavior in P2X7R-KO mice in the FST and the reduced use of anti-depressants.122 Numerous studies have confirmed the anti-depressant effects of P2X7R antagonists, such as blue brilliant G, 54,55,61 A-438079 61,62 and A-804598.53 Apart from these, selective brain penetrant P2X7R antagonist JNJ-54175446 and JNJ-55308942 have been transitioned into clinical placebo-controlled phase II studies to evaluate their effects in major depression disorder.124 However, hippocampal administration of P2R agonists injection (ATP 100 nmol/rat or BzATP 10 nmol/rat) for 3 weeks resulted in depressive-like behavior similar to that in stress exposure.61 Many researchers have also explored the possible mechanisms through which P2X7R may be involved in depression, including neuroinflammation, neuroexcitability, pyroptosis, etc. P2X7R stimulation has also been reported to increase the release of glutamate and γ-aminobutyric acid (GABA) 121,125,126 while reducing the uptake of these transmitters, 127 leading to excitatory toxicity.129 Pereira et al. found that when exposed to stress, P2X7R may be coupled with neuronal nitric oxide synthesis in the prefrontal cortex, leading to depressivelike behavior by promoting the release of NO in the limbic region of the brain, 130 which is consistent with the evidence that inhibition of NO synthesis can produce anti-depressant-like behavior.131 Chai et al. found that salidroside profoundly mediated corticosterone or lipopolysaccharide-induced depressive behavior and the improved synaptic plasticity by upregulating the expression of BDNF gene through P2X7/NF-κB/NLRP3(NF-κB, nuclear factor kappa-B; NLRP3, NOD-like receptor thermal protein domain associated protein 3) mediated pyroptosis. 132 conclusion, there is substantial evidence implying that P2X7R regulates the inflammatory body NLRP3/NLRP1, glutamate, GABA, NO release, and BDNF, and these mechanisms involve inflammation, neuroexcitability, pyroptosis, and more.
This part is contrary to our point that insufficient ATP release could lead to depression and that ATP supply may alleviate depression.The possible explanation is that high concentrations of eAT-P(EC 50 > 100 μM)act as a danger signal sensed by P2X7R, 133  Additionally, the released eATP can be rapidly converted into adenosine by ecto-nucleosidases in less than a minute, 134 actually in milliseconds within synapses. 135Both A1 receptors and A2a receptors have been found to play an important role in depression. 63,64,117,118wever, our understanding of the intricate interplay between adenosine and various other neuromodulation systems remains limited, and we know even less about the interaction between ATP and adenosine signaling-two sister worlds that seem to avoid looking into each other. 136Nonetheless, it is increasingly recognized that abnormal expression of eATP and regulation of ATP receptors may lead to depression 137 and eATP could be a novel therapeutic target for depression. 138

| ATP+P2X2R and depression
P2X2 receptors are ligand-gated ion channels that open a cationselective pore in response to ATP binding to its large extracellular domain. 139Gao Tianming's team discovered that decreased ATP release from astrocytes led to depressive-like behaviors in mice. 41nversely, exogenous ATP administration or endogenous activation of astrocytes promoted ATP release, revealing the potential cellular and neural circuit mechanism of ATP regulation of depressive-like behaviors.ATP deficiency via the P2X2 receptor inhibits GABAergic interneurons, resulting in reduced GABAergic inhibition of mPFC neurons projecting to LHb. 57 Consequently, the activity of this pathway is correlated with the occurrence of depressive-like behaviors. 57Additionally, their research also suggests that astrocytesderived ATP may regulate autism-like disorders through the P2X2 receptor. 140o and colleagues have identified that P2X2 receptors are increased in the mPFC of susceptible mice in Chronic Social Defeat

Stress (CSDS). Conditional knockout of P2X2 receptors in pyramidal
neurons promoted resilience of chronic stress-induced depressivelike behaviors, whereas pyramidal neurons -specific gain of P2X2 in the mPFC increased vulnerability to depressive-like behaviors. 58wer log-transformed P2X2 gene expression was observed in the peripheral blood of MDD than in that of Healthy volunteers, suggesting a significant relationship between P2X2 mRNA expression and depression. 141 is shown that activation of postsynaptic P2XRs by exoge- P2X2Rs trigger dynamin-dependent internalization of AMPA receptors(AMPARs), leading to the reduced surface AMPARs in dendrites and at synapses.The alteration of AMPAR requires calcium influx through opened ATP-gated channels and phosphatase or calciumcalmodulin-dependent protein kinase II (CamKII) activities. 142These findings indicate that postsynaptic P2XRs play a critical role in regulating the surface expression of AMPAR, thereby regulating the synaptic strength. 142 is reported that both P2X2 and P2X4 interact with NMDARs in an inhibited manner. 143Ca 2+ entry through P2X2Rs or P2X4Rs is sufficient to trigger internalization of AMPARs in vitro.However, ATP-induced enduring decrease of miniature excitatory postsynaptic currents (mEPSCs) and ATP-induced AMPAR internalization in hippocampal neurons are blocked by pyridoxalphosphate-6azophenyl-2′,4′-disulphonic acid. 142 can be inferred that ATP might influence MDD through two distinct pathways involving P2X2 receptors.Firstly, ATP is believed to regulate GABA synaptic transmission, a process that has been proven to modulate autism spectrum disorder-like behavior. 140condly, ATP plays a significant role in the function, regulation, and downstream signaling of calcium channels, 144 contributing to depression. 145In conclusion, the regulation of ATP on P2X2 receptor may be a target for the treatment of mental diseases such as depression.

| ATP+P2X4R and depression
The P2X4 receptor (P2X4R) is an ATP-gated cation channel that is highly permeable to Ca 2+ and is widely expressed in neuronal and glial cell types throughout the central nervous system.A growing body of evidence indicates that P2X4R plays key roles in several central disorders. 146Among the seven P2X subunits, P2X4R stands out as the most widely distributed in various cell types throughout the body. 147Nevertheless, P2X4R expression in the brain is sparse 148 and its contribution to the synaptic modulation in normal conditions remains debated. 99,149pression, bipolar disorder, schizophrenia, and anxiety are neuropsychiatric disorders characterized by impaired dopamine (DA) homeostasis.As previously reported, P2X4R has been implicated in the regulation of DA homeostasis and sensory-motor gating 150 and there is increasing evidence implying that P2X4R plays a critical role in psychiatric disorders. 59,60,151Consistent with an involvement of P2X4 receptors, ivermectin (P2X4-potentiating drug 152 ) has been found to induce anxiolytic-like and depressive-like behavior in mice. 60Remarkably, data from a new conditional P2X4 internalization-defective knock-in mouse, namely P2X4mCherryIN, which displays an increase in the number of P2X4 receptors at the surface of targeted cells, further supports the relation between neuronal P2X4 and anxiety-like behavior as well as in memory. 153The anxiolytic effects resulting from a selective increase in surface P2X4 density in excitatory forebrain neurons of mice further emphasize the role of neuronal P2X4 in anxiety. 153Another study, focusing on the role of P2X4 in ischemic stroke using P2X4-KO mice, has shown that P2X4 deletion predisposes animals to chronic depression-like | 9 of 16 behavior after stroke. 59Significantly in this context, a recent study revealed that the potentiation of P2X4 by ivermectin leads to DA hyperactivity and the disruption of information processing, possibly due to the potential perturbation of the interaction between P2X4 and DA receptors. 154Collectively, these findings suggest that P2X4 antagonists could serve as novel anti-psychotic treatments for psychiatric disorders arising from sensorimotor gating impairments linked to the disruption of DA homeostasis.
In conclusion, these findings provide further evidence for the connection between eATP and depression and the regulation of ATP on the P2X4 receptor may be a target for the treatment of mental diseases such as depression.

| ATP+ adenosine and depression
eATP can be converted into adenosine by ecto-nucleotidase in milliseconds 135 in a channeling manner. 134The four adenosine receptor subtypes A1, A2A, A2B, and A3 are G-protein coupled.
Typically, A1 and A3 receptors in brain couple to the Gi/o family of G-protein, which inhibit the synthesis of cyclic AMP. 155The concentration of extracellular adenosine in the brain is determined by the hydrolysis of ATP released from neurons or astrocytes and by transport through equilibrative nucleoside transporters. 156Under neuropathological conditions such as ischemia, trauma, excitotoxicity, neurodegeneration, neuroinflammation, and epilepsy, the extracellular concentration of adenosine in the brain can rise rapidly from nanomolar to micromolar levels, which can have both beneficial and detrimental effects on the progression of the illness. 157Among these receptors, A1 receptors are most abundant and homogenously distributed in the brain, providing an inhibitory and general neuroprotective "tone." 158Neuronal and glial adenosine A2A receptors play a critical role in neuropsychiatric disturbances. 157Both A1 receptors and A2A receptors have been found to play an important role in depression. 63,64,117,118th A1R and A2AR modulate hippocampal glutamate release and NMDA-dependent LTP but aging affects the density of both A1R and A2AR. 105It is observed that adenosine modulates hippocampal LTP through activation of adenosine A1R mostly only in young animals than in adult animals and A2 receptors in synaptic plasticity are mostly observed in aged rather than young animals. 105What's more, A1 receptors in synaptic plasticity are under tight repression by A2A receptor engagement, and A2AR could play a key role in dampening A1R during high-frequency induction of hippocampal LTP. 159e beneficial effects of 12-hour sleep deprivation in experimentally induced depressive-like behavior of mice require astrocytic signaling to A1 receptors, which may provide a novel pathway for the development of anti-depressants. 118[65] The impact of a non-toxic concentration of caffeine on synaptic transmission and plasticity in excitatory synapses of the mouse hippocampus is critically dependent on the presence of endogenous extracellular adenosine. 160The antagonism of adenosine receptors is indeed the main mechanism operated by non-toxic concentrations of caffeine to modify information processing in neuronal networks while directly excluding a role for GABA or ryanodine receptors as targets of caffeine at non-toxic concentrations. 160[163] It is worth noting that epidemiological studies have revealed a relationship between caffeine consumption and reduced risk of depression 164 and some researches have demonstrated the use of caffeine as a self-medication among depressed patients. 165However, in human data, the therapeutic effects of caffeine depend upon the administered dose, as high doses of caffeine and theophylline not only do not improve depressive symptoms, but can in fact promote anxiety. 166,167Therefore, it is advisable for individuals with depression and anxiety syndromes to take caffeine in appropriate doses.
eATP can be converted into adenosine quickly.We think that it is hard to distinguish between ATP and adenosine pathways.
Notably, brain noxious stimuli trigger a sustained increase of eATP, which plays a key role as a danger signal in the brain.The concept of ATP as a danger signal implies not only the release of ATP but also the involvement of purinergic receptors in brain disorders, which has mostly been documented for P2X7R, P2Y1R, and A2AR. 168tabolic stress, characteristics of brain dysfunction, is associated with increased extracellular levels of both ATP and adenosine, both of which act as danger signals in the brain with potential deleterious effects.It is unlikely that any experimental setting can isolate the contribution of ATP from that of adenosine and the different roles fulfilled by ATP and adenosine. 136Therefore, we think that it is not rigorous and incomplete to discuss the role of eATP in depression in isolation from the contribution of adenosine and P2X receptors.
Additionally, high levels of adenosine reflect high energy expenditure, leading to the reduced levels of eATP.The adenosine A2A agonist CGS21680 has been reported to decrease the affinity of dopamine D2 receptors for dopamine. 169In the context of mood depression, A2A receptors could also control the release of glutamate 170 and NMDA 106 receptor function.In contrast to the particular interaction in the basal ganglion, A2A receptors could modulate the dopamine D2 receptor-mediated inhibition of synaptic transmission in the mouse prefrontal cortex. 171 have limited knowledge about the intricate interplay of adenosine with other various neuromodulation systems, and even less about the interplay between ATP and adenosine signaling, two sister worlds that seem to avoid looking into each other. 136 summary, we think that the elevated levels of adenosine in brain tissue can lead to depression, irritability and fatigue, and replenish energy can convert adenosine to adenosine triphosphate (ATP), finally alleviating depressive symptoms.
Given that currently available anti-depressants primarily impact monoaminergic signaling, specifically by inhibiting the reuptake of serotonin (5-hydroxytryptamine) and norepinephrine (NA), exploring alternative molecular targets other than monoamines may help identify biomarkers for depression.This could lead to the development of drugs with better therapeutic efficacy.Currently, some of the potential pharmacological treatments for depression include mood stabilizers, 172 A2A receptor antagonists, 173,174 ATP-sensitive P2X7 receptor antagonists, 54,62 P2X2 receptor agonists, 57 ATP-sensitive potassium channel opener etakalin, 175 182 is also a possible therapeutic avenue for depression therapy.CD39 is an enzyme that is responsible, together with CD73, for a cascade converting ATP into ADP and cyclic AMP. 183Hippocampal CD39 has been involved in depressive-like behavior induced by CSDS through hydrolyzing eATP.This suggests that CD39 could be a promising new target for the treatment of depression, which can be realized by CD39 gene silencing/ pharmacological inhibition or ATP supplementation. 48Moreover, increased synaptic ATP release, coupled to CD73-mediated formation of extracellular adenosine, could contribute to mood and memory dysfunction triggered by repeated restraint stress. 49erefore, interventions aimed at decreasing ATP release and CD73 activity could be novel strategies to mitigate the burden of repeated stress. 49Other studies have demonstrated that the downregulation of Panx1 in CSDS models results in the deficiency of eATP in mPFC, leading to depressive-like behaviors.This suggests that Panx1 may be a key target for anti-depressant therapy, particularly in the context of mefloquine-induced depression. 50rthermore, some anti-depressant drugs such as SSRIs, have been discovered to inhibit the activation of microglia and promote the release of ATP, BDNF, and vascular endothelial growth factor from astrocytes to restore neurogenesis. 47

| DISCUSS ION
Several pieces of evidence have indicated that decreased ATP release or increased ATP catabolism may be important factors involved in the development of depression.On the other hand, elevated eATP levels can exert an anti-depressant effect, eATP supply can exert an anti-depressant effect, providing a new strategy for the treatment of depression.However, while reviewing numerous studies, we encountered some contradictory findings.For instance, the activation of the P2X7 receptor has been confirmed to be closely linked to depression, and its activation typically requires a high concentration of ATP (EC50 ≥ 100 μmol L −1 ), often due to exposure to stressful conditions.In contrast, many other studies suggest that insufficient ATP release from astrocytes may lead to depression, leaving us puzzled about ATP's actual role in depression.First, it is important to recognize that stress is a complex process, and the relationship between stress and depression is not clear.Acute exposure to stressors could result in depression. 42,46When exposed to acute stressors, microglial cells can be activated, leading to ATP release (combined with P2X receptors) and the activation of the inflammatory response.The inflammatory response is known to be associated closely with depression.Conversely, chronic stress is often accompanied by mitochondrial structural damage, 58,184,185 potentially leading to the reduced ATP production and depression.
Currently, chronic stress models are predominantly used to study depression, and the existence of mitochondrial structural damage has been confirmed, along with an increasing body of research demonstrating the role of reduced ATP release from astrocytes in depression.Secondly, some articles have shown that ATP release from astrocyte could play an important role in the anti-depressant effects.In microglia, the interaction between ATP and purinergic receptors may facilitate the activation of downstream inflammatory responses, potentially leading to depressive-like behaviors.We speculate that these discrepancies may be attributed to variations in eATP concentration.Considering the mounting evidence supporting the role of ATP reduction in depression, it is reasonable to believe that the modulation of eATP levels or the inhibition of its degradation may emerge as a novel depression treatment strategy in the future.This approach could be particularly beneficial for patients who do not respond to monoamine anti-depressants.
To conclude, a growing body of researches lends the credibility to the notion that eATP plays an important role in depression.The development of drugs that increase eATP levels may be a promising strategy for treating depression in the future.However, several challenges need to be addressed: (1) Although there is increasing evidence implying that the dysregulation of eATP is related to the pathophysiology of depression, the cellular and neural circuitry mechanisms underlying how ATP regulates depressive-like behavior require further investigation.
Therefore, a large number of studies are needed to elucidate the role of eATP in depression.
(2) ATP has demonstrated a short duration of anti-depressant effects in mice with a high efficiency, 41 but it remains to be seen whether this effect could translate to human.To establish the precise correlation between eATP and depression and the feasibility of using eATP to treat depression in humans, numerous researches involving human brain samples are warranted.
(3) More targeted anti-depression drugs, such as P2X2 receptor agonists, need to be developed.
leading to the occurrence of depression.In this context, we emphasize that the concentration of eATP may contribute to different results.It is possible that within the normal range of ATP concentration, insufficient ATP release could lead to reduced eATP concentration, ultimately resulting in the development of depression.eATP supply may alleviate depression.However, in the condition of high eATP concentration (EC50 ≥ 100 μmol •L − 1 ), P2X7R is bound by eATP, activating the inflammatory pathway and leading to depression.
nous ATP or noradrenaline-dependent glial release of endogenous ATP decreases the amplitude of miniature excitatory postsynaptic currents and α-amino-w3-hydroxy-5-methyl-4-isoxazolpropionate (AMPA)-evoked currents in cultured hippocampal neurons.It has also been observed a P2X-mediated depression of field potentials recorded in the hippocampal CA1 region from brain slices.142

| 11 of 16 WANG 5 |
et al.In summary, many studies have shown that abnormal ATP release especially from astrocytes is associated with depression.In the future, more research about depression is needed to explore the relationship of eATP released not only from astrocytes but also from neurons, synaptic, and so on.What's more, considering the interactional complexity of ATP and purinergic signaling pathways, we need to specify the role of eATP rather than adenosine and P2X receptors in depression(although these two sisters [ATP and adenosine] worlds that seem to avoid looking into each other), aiming to increase the persuasive effects of eATP in depression.Most of the researches are confined to animal studies at present.Therefore, what we need to do is to conduct a large number of studies to clarify the mechanism of the relationship between eATP and depression, carry out more clinical studies on the treatment of depression patients with ATP, and develop targeted drugs to guide the treatment of depression.CON CLUS ION Depression is a severe mental disorder whose underlying mechanisms remain inadequately understood.Considering that a substantial portion of individuals with depression do not respond to traditional monoamine-based anti-depressant there is an urgent imperative to delve deeper into intricacies of depression mechanisms.Several studies have demonstrated that eATP may play a crucial role in depression, especially from the angle of ATP release from astrocytes.We present the point that the reduction in ATP release could serve as a pivotal role in the pathogenesis of depression.However, there are few researches on the mechanism of eATP and depression, and most of them are confined to animal studies at present.Nonetheless, the research on the interplay between eATP and depression remains limited, predominantly confined to animal studies at present.Therefore, further research is needed to elucidate the mechanisms underlying the relationship between eATP and depression.This would entail conducting a substantial number of clinical studies involving ATP treatment for individuals with depression, ultimately paving the way for the development of targeted medications designed to guide the treatment of depression.AUTH O R CO NTR I B UTI O N S Kaixin Wang and Shiqian Huang contributed substantially to the article concept and manuscript writing.Kaixin Wang, Shiqian Huang, Daan Fu, Xinxin Yang, Lulin Ma, Tianhao Zhang, Wenjing Zhao, Daling Deng, Yuanyuan Ding, Yanyan Zhang, and Li Huang retrieved literature and reviewed themanuscript.Xiangdong Chen revised and approved the final version before submission.All the listed authors have participated actively in the study, and have read and approved the final manuscript.ACK N OWLED G M ENTS This work was supported by the National Key Research and Development Program of China (grant 2018YFC2001802 to X. Chen); National Natural Science Foundation (grant 82071251 to X. Chen); Hubei Province Key Research and Development Program (grant 2021BCA145 to X. Chen).No conflicts declared.

5 of 16 WANG et al. 2 | NEUROLOG IC AL MECHANIS M OF e ATP IN DEPRE SS I ON 2.1 | Insufficient ATP release from astrocytes may lead to the occurrence of depression
Studies about the relationship between eATP and depression in various animal models.
[102][103][104]extracellular synaptic ATP on synaptic plasticity are a composite of effects of P2 and P1(adenosine) receptors.Postsynaptic responses to ATP are mediated by metabotropic P2Y and ionotropic P2X receptors, which are abundantly expressed in neural cells.Strong impacts of P2X receptors on synaptic plasticity arise from their high calcium permeability, capability to interact with other receptors, and widespread exocytotic release of ATP from central neurons and glia.99Generally,P2X-mediatedsynapticcurrents are not very large, rarely exceeding 50-100 pA in amplitude and accounting for 5%-15% of the synaptic currents mediated by glutamate.Nonetheless, the ATP-mediated synaptic transmission can still be functionally significant.This is especially noteworthy because the activation of postsynaptic P2X receptors can facilitate Ca 2+ entry at resting membrane potentials.99TheApplication of ATP triggers substantial Ca 2+ signals in central neurons, which are mediated by Ca 2+ entry through P2X receptors/voltage-gated Ca 2+ channels and by P2Y-mediated Ca 2+ release from intracellular store.100,101It is widely reported that cytoplasmic Ca 2+ signals are important for the synaptic potentiation and depression.[102][103][104] Synaptic plasticity damages play a crucial role in the onset and development of depression, especially in the hippocampus, which is more susceptible to stress and the most frequently studied brain region in depression. 93 7 of 16 WANG et al.